The present invention relates to a plating apparatus and a method of preventing a substitute deposition.
The plating apparatus comprises an anode, a cathode and a plating solution. The anode serves as a plating-metal source, wherein the anode comprises an Snxe2x80x94Pb alloy in the form of a plate or a ball contained in a basket or a bag. Surfaces of lead frames are activated in a pre-treatment to allow the lead frames to serve as a cathode. The plating liquid contains an organic acid or an additive. The lead frames are dipped into the plating liquid. A voltage is applied across the cathode and the anode to ionize Sn and Pb at the anode so that Su-ions and Pb-ions, mainly cations, are dissolved into the plating solution. On the cathode, The Sn ions and the Pb ions receive electrons, whereby an alloy coating film is formed on the surfaces of the lead frames as the cathode.
In the plating method using the Sn-based alloy except for Snxe2x80x94Pb alloy, a difference in tendency of ionization, which is inherently possessed by each metal element, causes a substitute-deposition whereby a wettability and a plating-adhesiveness are dropped. The lead frames are dipped into the plating solution without receiving application of the voltage, for which reason the substitute-deposition is caused to form a metal film on the surface of the lead frame. Further, if during the plating process or after the formation of the plated film, the plating solution enters into an electroless state, then the substitute-deposition is caused to form the substitute-deposited coating film which is different from the intended alloy metal coating film. The substitute-deposited coating film deteriorates the solder-wettability and the adhesiveness with the base martial. The substitute-deposited coating film makes unstable the metal ratios and deposited composition of the plating solution. During the plating process with a current, motions of the metal ions are controllable by externally applying a voltage, whilst if the product or the anode remains dipped in the solution without any current application, then the substitute-deposition is likely to be caused. Namely, if no current is applied to the product or the anode in a time-period, then the substitute-deposition is likely to be caused. Particularly, in the lack-plating system, after a lack has completely been dropped with its gravity into the bottom, a current application is first commenced, for which reason the lack-plating system is engaged with the above problem.
In the lack-less plating system, if the current application is accidentally discontinued, then the substitute-deposition is also likely to be caused.
Further, the anode remains dipped in the plating solution even after the plating process has been finished, for which reason if the substitute-deposition is caused on the anode, it is necessary to remove pick-up the anode to receive the substitute-deposition coating film form the anode. This process is inconvenient and danger.
In the above circumstances, it had been required to develop a novel plating apparatus and a method of preventing a substitute-deposition free from the above problem.
Accordingly, it is an object of the present invention to provide a novel plating apparatus free from the above problems.
It is a further object of the present invention to provide a novel plating apparatus for plating an alloy of plural metal elements different in tendency of ionization.
It is a still further object of the present invention to provide a novel plating apparatus for plating an Snxe2x80x94Bi alloy of plural metal elements Sn and Bi different in tendency of ionization.
It is yet a further object of the present invention to provide a novel plating apparatus for plating an Snxe2x80x94Cu alloy of plural metal elements Sn and Cu different in tendency of ionization.
It is yet a further object of the present invention to provide a novel plating apparatus for plating an Snxe2x80x94Ag alloy of plural metal elements Sn and Ag different in tendency of ionization.
It is yet a further object of the present invention to provide a novel plating apparatus for plating an Snxe2x80x94Zn alloy of plural metal elements Sn and Zn different in tendency of ionization.
The first present invention provides a plating apparatus comprising: a plating bath filled with a plating solution; at least an anode in the plating solution; at least a plating object which serves as a cathode in the plating solution, so that the at least plating object is distanced from the at least anode; and at least a dummy cathode in the plating solution, so that the at least dummy cathode is applied with voltage to suppress a substitute-deposition of metal ions in the plating solution.
The second present invention provides a method of suppressing a substitution-deposition of metal ions in a plating solution filled in a plating bath which has at least an anode, at least a plating object serving as a cathode and at least a dummy cathode, and the plating solution containing at least two kinds of metal ions which are different in ionization tendency from each other, wherein when no voltage is applied to the plating object, the at least dummy cathode is applied with a voltage to suppress a substitute-deposition of metal ions.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.